Mass spectrometry is an analytical methodology used for quantitative elemental analysis of samples. Molecules in a sample are ionized and separated by a spectrometer based on their respective masses. The separated analyte ions are then detected and a mass spectrum of the sample is produced. The mass spectrum provides information about the masses and in some cases the quantities of the various analyte particles that make up the sample. In particular, mass spectrometry can be used to determine the molecular weights of molecules and molecular fragments within an analyte. Additionally, mass spectrometry can identify components within the analyte based on a fragmentation pattern.
Analyte ions for analysis by mass spectrometry may be produced by any of a variety of ionization systems. For example, Atmospheric Pressure Matrix Assisted Laser Desorption Ionization (AP-MALDI), Field Asymmetric Ion Mobility Spectrometry (FAIMS), Atmospheric Pressure Ionization (API), Electrospray Ionization (ESI), Atmospheric Pressure Chemical Ionization (APCI) and Inductively Coupled Plasma (ICP) systems may be employed to produce ions in a mass spectrometry system. Many of these systems generate ions at or near atmospheric pressure (760 Torr). Once generated, the analyte ions must be introduced or sampled into a mass spectrometer. Typically, the interior portions of a mass spectrometer are maintained at high vacuum levels (<10−4 Torr) or even ultra-high vacuum levels (<10−7 Torr). In practice, sampling the ions requires transporting the analyte ions in the form of a narrowly confined ion beam from the ion source to the high vacuum mass spectrometer chamber by way of one or more intermediate vacuum chambers. Each of the intermediate vacuum chambers is maintained at a vacuum level between that of the proceeding and following chambers. Therefore, the ion beam transports the analyte ions transitions in a stepwise manner from the pressure levels associated with ion formation to those of the mass spectrometer.
In most applications, it is desirable to transport ions through each of the various chambers of a mass spectrometer system without significant ion loss. Ion transport is usually accomplished using an ion guide that moves ions in a defined direction in a narrow beam. Ion guides typically utilize electromagnetic fields to confine the ions radially (x and y) while allowing or promoting ion transport axially (z).
Ion guides also employ repellent inhomogeneous radio frequency (RF) fields to create electric pseudo-potential wells to confine the analyte ions as they travel through the guide, and a voltage potential between the input and output ends of the device to move ions through the guide. However, prior art devices are prone to “RF droop” (i.e., areas of reduced RF) if high resistance multipole rods are used. As such, in many ion guides ions may become stalled (and/or filtered) as they are transported through the guide.
Thus, there is still a need for ion guides that efficiently transport ions without significant ion loss or power dissipation.